The present invention generally relates to semiconductor devices, and more particularly to a process for writing a semiconductor pattern on a substrate by a focused electron beam.
The submicron patterning process is one of the most important targets in the current fabrication technology of semiconductor integrated circuits. With the submicron patterning, one can increase the operational speed of logic devices significantly. In the semiconductor memories, on the other hand, the increase of the integration density associated with the submicron patterning directly relates to the increase of storage capacity of information.
In the optical exposure process used conventionally in the patterning of semiconductor devices, a pattern size of about 0.3 .mu.m sets the lower limit that can be achieved even though combined with the phase shift mask. On the other hand, the electron beam exposure system can write a pattern on a semiconductor substrate with a pattern size of 0.1 .mu.m or less. There, one can achieve a pattern alignment with an error of less than 0.05 .mu.m. Thus, the electron beam exposure system is expected to become a key facility in the fabrication of future integrated circuits and various quantum devices that use the quantum mechanical effect of electrons and holes.
In the electron beam exposure system, on the other hand, there exits a problem, arising from the principle of the exposure, in that the throughput is low as compared with the optical exposure systems. It should be noted that, in the electron beam exposure systems, one has to write a device pattern on the substrate by deflecting a single electron beam. In order to accelerate the speed of exposure, various proposals have been made so far, including the use of the block exposure process wherein the electron beam is shaped variously by passing through a stencil mask or the use of the blanking aperture array wherein the electron beam is shaped into a dot pattern corresponding to the device pattern by passing through a blanking aperture array.
In the final stage of the electron beam exposure process, the electron beam thus shaped is focused on the surface of a substrate or wafer by an objective lens and causes a chemical reaction in an electron beam resist that is applied on the surface of the substrate. In this process, the electron beam is deflected to a desired location of the substrate surface. It should be noted that the range of deflection of the electron beam is limited typically on the order of several millimeters and thus a dimension which is much smaller than the size of the substrate. Thus, the exposure of the substrate is achieved by dividing the surface of the substrate into a number of strip-like zones extending parallel with each other with a width of a few millimeters and transporting the substrate in the elongated direction of the strip-like zones.
The deflection of the electron beam is achieved by energizing an electromagnetic deflector and an electrostatic deflector, wherein the electromagnetic deflector covers a range of several millimeters while the electrostatic deflector can cover only a range of several hundred microns. In exchange, the electromagnetic deflector exhibits a relatively slow response due to the delay caused by the inductance, while the electrostatic deflector exhibits a very fast response.
In the foregoing deflection process, each zone is divided into a number of parallel bands extending perpendicularly to the elongating direction of the zone, and each zone is divided into a number of sub-fields, wherein each zone may have a width of 2 mm and each sub-field may have a size of 100 .mu.m.times.100 .mu.m. The exposure of the device pattern is achieved for each sub-field by deflecting the electron beam by the electrostatic deflector while moving the substrate in the elongated direction of the strip-like zones continuously. There, the jump from one sub-field to an adjacent sub-field in the band is achieved by the electromagnetic deflector.
FIG. 1 shows a conventional electron beam exposure system that achieves the foregoing exposure operation.
Referring to FIG. 1, the electron beam exposure system includes a CPU 1 and a data storage device such as a magnetic disk device 2 or a magnetic tape device 3, wherein the devices 2 and 3 are used to store pattern data corresponding to a device pattern of a semiconductor device to be written on a substrate. The CPU 1, the magnetic disk device 2 and the magnetic tape device 3 are connected commonly to a system bus 4, and the CPU 1 reads out the pattern data from the magnetic disk 2 or from the magnetic tape 3 on the system bus 4. The pattern data thus read out on the system bus 4 is then transferred via an interface circuit 5 to a data memory unit 6 and simultaneously to a stage controller 7.
The electron beam exposure system further includes an evacuated column 8 as usual, and there is provided an electron gun 9 at the top part of the column 8 for producing an electron beam. The electron beam thus produced by the electron gun 9 is focused on a substrate 18 that is held on a movable stage 17 after passing through various electron lenses 10, 12 and 15 as well as after being deflected by electrostatic deflectors 13 and 14, wherein the electron lens 15 acts as the objective lens for focusing the electron beam on the surface of the substrate 18. The deflector 11 is used for blanking control together with the electron lens 10 and a blanking aperture, not illustrated in FIG. 1, and causes the turning-on and turning-off of the electron beam. The electron lens 12 on the other hand is used in combination with the deflector 13 and a shaping slit (not illustrated) for shaping the electron beam into a desired beam shape.
The electron beam thus shaped is deflected by the electrostatic deflector 14 and is moved over the surface of the substrate 18 when focused thereon by the electron lens 15. Further, there is provided an electromagnetic deflector 16 for deflecting the focused electron beam over a wide range of the substrate surface. It should be noted that the electrostatic deflector 14 provides the deflection of the electron beam over a limited area, that is smaller than about 100 .mu.m.times.100 .mu.m, with a high speed of about 0.6 .mu.s/3 .mu.m. On the other hand, the electromagnetic deflector 16 provides the deflection over a large area, as large as 1 mm.times.1 mm, though with a limited speed of about 2-30 .mu.s/100 .mu.m.
In operation, the pattern data stored in the data memory unit 6 is read out by a pattern generator 19. The pattern data thus produced is then supplied to a pattern compensation circuit 20 that extracts a blanking control signal from the pattern data and supplies the same to the electrostatic deflector 11 via an amplifier 21. Simultaneously, the circuit 20 produces beam shape control data specifying the beam shape that is to be used for the shot. It should be noted that the beam shape control data is produced consecutively in correspondence to the shot and supplied to the ,electrostatic deflector 13 after conversion thereof to an analog signal in a D/A converter 22 and subsequent amplification by an amplifier 23. Further, in correspondence to each shot, the pattern compensation circuit 20 produces deflection control data and supplies the same to the electrostatic deflector 14 after conversion thereof to an analog signal in a D/A converter 24 and subsequent amplification in an amplifier 25. Furthermore, the pattern compensation circuit 20 produces another deflection control data and supplies the same to the electromagnetic deflector 16 after conversion thereof to an analog signal in a D/A converter 26 and subsequent amplification in an amplifier 27.
As already noted, the electrostatic deflector 14 and the electromagnetic deflector 16 cause movement of the electron beam over the surface of the substrate 18. During this deflection, the substrate 18 is moved in the X-Y plane in a predetermined direction by the stage 17 that in turn is driven by a control signal produced by the stage controller 7. Generally, the speed of movement of the stage 17 is set to a constant value. Thereby, the exposure is achieved along a number of parallel zones extending in the moving direction of the substrate 18 as shown in FIG. 2.
Referring to FIG. 2, a typical zone is represented by the shading. It should be noted that there are a number of zones defined on the surface of the substrate 18 so as to extend side by side. As already noted, the zone is divided into a number of bands that extend in the direction perpendicular to the elongated direction of the zone, and the exposure is achieved in correspondence to an exposure region 101 defined with respect to an optical axis of the electron beam exposure system as will be described later while moving the substrate as indicated by the arrow A in FIG. 2.
FIG. 3 shows an example of the division of the zone of FIG. 2 into the bands.
Referring to FIG. 3 which illustrates a part of a zone 100, the zone 100 is defined to extend in the moving direction of the substrate represented by an arrow A and includes a number of parallel bands B.sub.1 -B.sub.n, wherein a typical band B.sub.i is represented by the shading. Each band extends in a direction perpendicular to the elongated direction of the zone 100 and has a width b.sub.1 -b.sub.n that may be constant or may be different for each band. The respective positions of the leading edge and the trailing edge of the band are represented as bp.sub.0, bp.sub.1, . . . . For example, the position of the leading edge of the band B.sub.i is represented as bp.sub.i-1 while the position of the trailing edge of the same band B.sub.i is represented as bp.sub.i. Further, each band is formed with a number of sub-fields F.sub.1 -F.sub.m that are aligned in the elongated direction of the band.
As already noted, the pattern is written, sub-field by sub-field, by an electron beam focused by an objective lens, wherein the electron beam is deflected inside the sub-field by the electrostatic deflector 14. On the other hand, the jump from one sub-field to a next sub-field is achieved by the electromagnetic deflector 16. During the exposure, the substrate 18 is moved in the direction A continuously. Thus, it is necessary to set the speed of movement of the substrate 18 such that the exposure of each band is completed before the band moves beyond the range that the electron beam can cover by the deflection. It should be noted that the time needed for the exposure may change in each sub-field and hence in each band, particularly in the devices that include an irregular device pattern such as microprocessors. More specifically, it will be understood that a complex pattern may need a large number of shots and hence a longer exposure time, while a simple pattern having a relatively reduced number of shots can be written in a short exposure time. In such a case, therefore, the time needed for the exposure is unpredictable unless evaluated in advance by analyzing the pattern to be written.
FIG. 4 shows an example where the exposure has failed.
Referring to FIG. 4, the vertical axis represents the position, measured in the direction A, of the bands B.sub.1 -B.sub.n defined on the surface of the substrate, and the horizontal axis represents the timing of the exposure of the band that is conducted in synchronization with the passage of the exposure region 101 (see FIG. 2). Further, lines s and t of FIG. 4 represents the relative movement of the foregoing exposure region 101 with respect to the bands defined on the substrate 18, wherein the line s corresponds to the leading edge of the exposure region 101 and defines the beginning of the interval in which the exposure of device pattern is possible by deflecting the electron beam under the control of the electromagnetic and electrostatic deflectors 14, 16. On the other hand, the line t corresponds to the trailing edge of the exposure region 101 and defines the end of the interval beyond which the exposure of device pattern by the deflection of the electron beam is not possible.
It should be noted that the lines s and t are separated from each other by a distance D that corresponds to the width of the exposure region 101. See FIG. 2 for setting of the exposure region 101 with respect the substrate 18. The width D may be set to about 1 mm or less in the moving direction A of the substrate, in correspondence to the range that can be covered by the deflection of the electron beam by the electromagnetic deflector without causing a substantial aberration. When the size D is set to 1 mm, the exposure region 101 includes therein about 10 bands each having a width b.sub.i of 100 .mu.m. In a preferred example, the size D is set three times as large as the width b.sub.i of the band. It should be noted that, during the time interval of the exposure region 101 defined between the line s and the line t, one can conduct the exposure of the sub-fields F.sub.1 -F.sub.m of the respective bands.
In FIG. 4, it will be noted that the exposure of the band B.sub.1 is achieved successfully in an interval .DELTA.t.sub.1 beginning at a time bt.sub.0 and ending at a time bt.sub.1. Immediately after the exposure of the band B.sub.1 is completed, the exposure of the next band B.sub.2 starts as shown by an interval .DELTA.t.sub.2. In this way, the exposure of the rest of the bands is achieved consecutively.
As can be seen in FIG. 4, the band that includes a complex pattern such as the band B.sub.3 needs a longer time .DELTA.t.sub.4 for the exposure than other bands, and the existence of such a complex band causes a delay in the timing of starting the subsequent exposure operation. Thereby, there occurs a case as in a band B.sub.8 where the exposure of the band extends beyond the trailing edge t of the exposure region. In such a case, the exposure of the band cannot be completed, and the exposure operation has to be interrupted, by stopping the movement of the substrate 18. However, such a interruption of the movement of the stage 17 in response to each band is difficult and undesirable in view of the finite inertia of the stage driving system. Further, the repeated interruption and reactivation of the movement of the stage 17 may cause a problem in the alignment of the device pattern on the substrate 18.